1. Field of the Invention
This invention relates to a sintered article for a heath to be used in an apparatus for continuous casting of a thin sheet and to a sheath using the sintered article. More particularly, it relates to a sintered article for a sheath to be used in an apparatus for continuous casting of a thin sheet, which apparatus is provided on the melt pouring side thereof with cylindrical surfaces as in a twin roll type or twin belt type apparatus which comprises two water-cooled rolls or two endless belts parallelly opposed to each other across a gap with the rotary shafts thereof laid horizontally and two sheath plates applied one each fast to the opposite lateral ends of the parallelly disposed water-cooled rolls or endless belts so as to give rise to an enclosure like a container on the parallelly disposed water-cooled rolls or endless belts and which effects continuous casting of a thin sheet by pouring a melt into the enclosure and rotating the two water-cooled rolls or endless bolts inwardly toward each other, and a sheath using the sintered article.
2. Description of the Prior Art
Heretofore, sintered articles of boron nitride (BN) and sintered articles of zirconium oxide (ZrO.sub.2) and alumina (Al.sub.2 O.sub.3) have been known as materials for the parts which, like a sheath used in an apparatus provided on the inlet side thereof with cylindrical surfaces and operated for continuous casting of a thin sheet, are required to possess stability to resist thermal shock, corrosion, and abrasion.
Incidentally, the sintered articles of BN show prominence in resistance to thermal shock and in lubricity on the one hand and betray low strength and deficiency in resistance to corrosion in the melt of steel such as stainless steel on the other hand. In contrast, the sintered articles of ZrO.sub.2 and Al.sub.2 O.sub.3 possess high strength and nevertheless suffer from the disadvantage that the sheaths using these sintered articles inflict scratches on the water-cooled rolls or endless belts which slide on the sheaths.
For this reason, it has been proposed to divide a sheath between the portion destined to contact the melt and the part destined to slide against the water-cooled rolls or endless belts and use the sintered article of BN or Si.sub.3 N.sub.4 and the like for the former portion and the sintered article of ZrO.sub.2 or Al.sub.2 O.sub.3 and the like for the latter portion (Japanese Patent Application Disclosure SHO 62(1987)-166,054 and Japanese Patent Application Disclosure SHO 60(1985)-162,557).
The conventional sheath of the aforementioned type which combines portions different in material as described above, however, it has the disadvantage that the sheath is complicated in construction and troublesome to produce because it possibly induces leakage of the melt unless the different materials being used are adapted to possess an equal thermal expansion coefficient and the portions different in material are fitted to each other exactly. Thus, the desirability of providing a sintered article for an sheath which produces the sheath consisting solely of the sintered article instead of a combination of different materials and excelling in resistance to corrosion, resistance to thermal shock, and resistance to abrasion and consequently providing a sheath using the sintered article has been finding enthusiastic recognition.